In comparison to steel, concrete is a very weak material in tension. It reacts poorly to shear forces which create significant tensile forces, typically along inclined planes running between exterior surfaces of a reinforced concrete member.
Without shear reinforcement, shear failure in reinforced concrete members is brittle and occurs without much warning. A shear failure generally takes place by widening of an inclined crack which propagates from the face of the concrete member which is in tension to the compression face. In comparison, a flexural failure of a reinforced concrete member is much more ductile and provides more warning prior to the failure of the flexural reinforcement because of the formation of cracks readily visible to the naked eye and the relatively large deflections of the concrete member.
Shear reinforcement in the form of stirrups and cross ties is provided to prevent shear failure. Stirrups resist tensile forces in reinforced concrete caused by: shearing in beams, corbels, bridge piers and walls; punching in slabs and walls; lateral expansion in columns; and splitting behind anchorages and below bearings, at points of concentrated loading.
A stirrup is typically a reinforcing bar bent in a "U", "L" or closed box shape. The ends of the bar are usually in the form of hooks. A reinforcing bar, running in a direction perpendicular to the plane of the stirrup, is commonly lodged inside the hooks or the bends of the stirrups. Stirrups in a flat concrete slab, for example, contribute to shear resistance by developing tensile forces in the vertical legs of the stirrup. These tensile forces arise when the stirrup leg is intercepted by a crack forming in the slab. However, such tensile forces cannot develop unless the stirrup leg is anchored effectively at both its ends to prevent it from being pulled out. This anchorage is provided by the bend of the stirrup at its comers or by the hooked ends. A small slip in this anchorage reduces the effectiveness of the stirrup. The slip prevents the tension in the short stirrup leg from reaching its yield strength, and so the full capacity of the stirrup is not realized.
Cross ties function in much the same way. A cross tie is a stirrup in the form of an "L" and is commonly provided with one hook at the upper end of the "L". A cross tie is sometimes made in the form of one straight bar with two hooks; but this is difficult to install.
Should the tension in a stirrup leg (or a cross tie) approach its yield strength, very high compressive stresses are developed and exerted on the concrete in contact with the inner face of the bend or hook. By virtue of the commonly used radii for such bends (and as allowed by the American Concrete Institute (ACI) Building Code and the Codes of other jurisdictions), these compressive stresses are sufficient to crash the concrete inside the bend, resulting in a measurable slip of the leg and dislocation of the hook. Such slip causes large strain losses in the leg and diminishes the stirrup's capacity to prevent the widening of a crack. The loss of strain, and hence the loss of force resisted by the stirrup leg, is large because the stirrup leg tends to be short, particularly in slabs and walls.
The above noted slippage has been reported in the Journal of American Concrete Institute (Vol. 77, No. 1, January/February 1980, pp. 28-35, by F. Seible, A. Ghali and W. H. Dilger) and in Bautechnik (Vol. 42, October 1965, by F. Leorkhardt and K. Walther (in German)).
Use of stirrups and cross ties also presents other problems: they are difficult to form properly; installing flexural reinforcement through rows of stirrups, often required in two orthogonal directions, is extremely difficult and time consuming; and stirrup congestion in high shear locations makes it difficult to pour and vibrate concrete. Consequently, given a choice, many designers would prefer omitting closed stirrups in reinforced concrete design.
Solutions to some of the above-noted problems associated with stirrups and cross ties have been proposed by the present inventors in Canadian Patent 1,085,642 issued Sep. 16, 1980 and U.S. Pat. No. 4,406,103 issued on Sep. 27, 1983, which describe stud shear reinforcement for flat concrete slabs. One form of this stud shear reinforcement comprises a plurality of spaced, substantially vertical steel rods fixed at the bottom to a flat supporting base plate. The top of each rod has an anchor head to provide anchorage of the reinforcement within the concrete slab. The anchor head is mechanically attached to the stem of the stud, usually by forging, cold forming or welding. This reinforcement has enjoyed wide acceptance and use in the construction industry.
A vertical stud of the prior patents which crosses a crack in a slab will prevent the crack from widening provided that no slip occurs, at least until the yield stress of the stud is reached. To avoid slippage, the anchor head must be sufficiently large so that the concrete behind (i.e. on the stem side of) the head does not crush while the tensile force in the stem of the stud remains below its yield strength. On the other hand, the size of the anchor head should not be so large as to make forging impossible or too costly, it should not complicate the placement of flexural reinforcement, nor should it interfere too much with the casting of concrete in congested areas. It has been generally accepted that an anchor head should have an area about 10 times the cross-section area of the intermediate stem of the stud to avoid crushing of concrete, depending on the quality and strength of the concrete used. In some circumstances the size of the anchor head necessary to avoid crushing may result in a clearance between adjacent anchor heads which is rather tight, making arrangement of the longitudinal bars needlessly inconvenient and difficult.
The studs of the prior patents are welded at a pre-set spacing to the elongate base plate prior to placement in concrete formwork. Such welding is rather expensive and slows production time of the stud shear reinforcement. The welding process is also difficult to do on-site, and hence the stud shear reinforcement is almost always produced off-site in a shop
What is desired therefore is a novel stud reinforcing system which overcomes the limitations of these other prior reinforcing systems. Preferably it should allow convenient off-site or on-site placement of studs at a desired spacing on a support element and avoid having studs welded at a pre-set spacing to a base plate. The support element should allow use of a reduced size of anchor head by providing confinement of highly stressed concrete behind the anchor head. It should be possible to use the stud reinforcing system to resist tension associated with shear and in situations where the full yield strength of the stud is needed to resist tension immediately behind the anchor head. It should also provide for the anchor heads of a stud to be arranged as close as possible to the external faces of a concrete member to maximize the length of the stud and its chances of intersecting cracks formed in the concrete member.